The disclosure relates to deposition of aerospace coatings. More particularly, the disclosure relates to electron beam physical vapor deposition (EB-PVD) of ceramic thermal barrier coatings (TBC).
The current state of the art thermal barrier coatings (TBC) are produced by EB-PVD with a line of sight deposition capability. Line-of-sight deposition occurs with the process done in a vacuum chamber at absolute pressures varying from 1×10−4 to 1.5e−2 Torr (0.013 to 2.0 Pa), as disclosed in Rigney, et al., U.S. Pat. Nos. 6,342,278 and 6,447,854. Vacuum levels below 1.5e−2 Torr (2.0 Pa) ensure robust and durable operation of the electron beam guns. At these pressures, the mean free paths of gas and vapor molecules in the vacuum chamber are at least as long as the size of the chamber. Thus, on average, gas and vapor molecules travel from the vapor source to the part being coated without colliding with other gas or vapor molecules. Thus, on average, depositing vapor molecule trajectories are straight lines, leading to line-of-sight deposition.
Because TBCs are critical to part life and operation, TBC thickness and coat zones are an input factor in part design. Today turbine vanes are typically manufactured as singlets to enable much more uniform TBC thickness to allow these parts to operate in higher temperature environments. However, a doublet (or triplet, etc.) has advantages in minimizing leakage losses between parts. There have been problems obtaining uniform coatings on areas of each doublet airfoil which become occluded by the other during deposition (see, e.g., U.S. Pat. No. 8,191,504 of Blankenship).
Similar TBC thickness issues occur on other parts as well including turbine blades. Optimizing deposition for the airfoil of a turbine blade results in low thickness build on the platform region. Complex manipulation today has improved the thickness distribution but an increase in non-line-of-sight (NLOS) deposition is desired for these components as well.
Modifications to the EB-PVD process are known in the prior art that enable NLOS deposition. The directed vapor deposition (DVD) process relies on supersonic jets surrounding the vapor source to improve NLOS deposition, as disclosed in U.S. Pat. No. 8,110,043 to Hass et al.